# strlen, strnlen_s

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< c‎ | string‎ | byte

Defined in header ``<string.h>`` |  |
size_t strlen( const char* str ); | (1) |
size_t strnlen_s( const char* str, size_t strsz ); | (2) | (since C11)

1) Returns the length of the given null-terminated byte string, that is, the number of characters in a character array whose first element is pointed to by str up to and not including the first null character.
The behavior is undefined if str is not a pointer to a null-terminated byte string.
2) Same as (1), except that the function returns zero if str is a null pointer and returns strsz if the null character was not found in the first strsz bytes of str.
The behavior is undefined if str is not a pointer to a null-terminated byte string and strsz is greater than the size of that character array.

  As with all bounds-checked functions, `strnlen_s` is only guaranteed to be available if __STDC_LIB_EXT1__ is defined by the implementation and if the user defines __STDC_WANT_LIB_EXT1__ to the integer constant 1 before including `<string.h>`.

### Parameters

str | - | pointer to the null-terminated byte string to be examined
strsz | - | maximum number of characters to examine

### Return value

1) The length of the null-terminated byte string str.
2) The length of the null-terminated byte string str on success, zero if str is a null pointer, strsz if the null character was not found.

### Notes

`strnlen_s` and `wcsnlen_s` are the only bounds-checked functions that do not invoke the runtime constraints handler. They are pure utility functions used to provide limited support for non-null terminated strings.

### Example

Run this code

    #define __STDC_WANT_LIB_EXT1__ 1
    #include <stdio.h>
    #include <string.h>

    int main(void)
    {
        const char str[] = "How many characters does this string contain?";

        printf("without null character: %zu\n", strlen(str));
        printf("with null character:    %zu\n", sizeof str);

    #ifdef __STDC_LIB_EXT1__
        printf("without null character: %zu\n", strnlen_s(str, sizeof str));
    #endif
    }

Possible output:

    without null character: 45
    with null character:    46
    without null character: 45

### References

- C23 standard (ISO/IEC 9899:2024):

  - 7.24.6.3 The strlen function (p: TBD)

  - K.3.7.4.4 The strnlen_s function (p: TBD)

- C17 standard (ISO/IEC 9899:2018):

  - 7.24.6.3 The strlen function (p: TBD)

  - K.3.7.4.4 The strnlen_s function (p: TBD)

- C11 standard (ISO/IEC 9899:2011):

  - 7.24.6.3 The strlen function (p: 372)

  - K.3.7.4.4 The strnlen_s function (p: 623)

- C99 standard (ISO/IEC 9899:1999):

  - 7.21.6.3 The strlen function (p: 334)

- C89/C90 standard (ISO/IEC 9899:1990):

  - 4.11.6.3 The strlen function

### See also

wcslenwcsnlen_s (C95)(C11) | returns the length of a wide string (function)
mblen | returns the number of bytes in the next multibyte character (function)
C++ documentation for strlen

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